Control circuit for motors



Nov. 1, 1960 l R. J. LINCOLN 2,958,815

coumor. cmcurr FOR MOTORS Filed March 31, 1958 2 Sheets-Sheet 1 Ala/5TLOWE? 54321 123 45 Zhwentor aawyfw, H 2M I g Gttoruegs.

Nov. 1, 1960 R. J. LINCOLN 2,958,815

CONTROL CIRCUIT FOR MOTORS Filed March 31, 1958 2 Sheets-Sheet 2 O a 2 24; a 10 1'2 wo. v00:

Jilanomegs.

United States CONTROL CIRCUIT FOR MOTORS Robert J. Lincoln,Westfield,'NJ., assignor to Square D i This invention relates to motorcontrol systems for direct current motors. adaptable to those useswherein a load is hoisted and lowered and wherein hoisting movement ofthe, load is limited when it has been hoisted beyond a certain position.

A specific example of such use is in a hoist where a load is hoistedduring hoisting rotation of the armature of a direct current motor andlowered during lowering rotation of the armature and wherein hoisting ofthe load beyond a certain position is limited by a limit trip switch toprevent hoisting the load into the hoist supporting structure. 7

Various systems are safely in operation to control hoist'n'iotors' sothat safe hoisting and lowering of the load at safe speeds and forsafedistances is obtained."

The systemv is particularly by causing an armature shunt circuitincluding a unidirectional resistance to become effective to limit thelowering speed to a safe value while lowering out of the limit switchzone, as the result of opening at least one of the usual normally closedcontacts of an overhoist limit switch.

'The above objects and others are provided by a motor operating systemhaving a first circuit, which, when energized by a source of energydirects current through the motor armature in a hoisting direction andthrough the motor field winding in a forward direction in series tocause hoisting rotation of the armature. Upon deener: gization of thefirst circuit by hoisting into a limit switch; a second circuit becomeseifective to dynamically brake the armature by directing current,generated by continued rotation 'of the armature in the hoistingdirection after such deenergization of the first circuit, through thearmaturein a lowering direction and through thefield winding,

in the forward direction. The system also includes a third circuitwhich, when energized by thesource of ene'rgy, directs current throughthe armature in the lowering direction and through the field winding inthe forward direction to effect safe lowering rotation of the armatureata slow speed. This third circuit includes a unidirectional orrectifier circuit shunting the armature.

The effective resistance of a rectifier to the flow of 7 currenttherethrough in its conductive direction is related Thesesystems includelimit switches and 'rnotor contr ol circuits. An example of a priorsystem is described in Patent No. 2,515,612, issued July 18, 1950, to H.J. Rathbun and entitled Motor Control System.

In a control system of the above identified patent, hoisting into thelimit switch automatically trips the limit switch and applies a dynamicbraking circuit to the motor to stop hoisting movement thereof. Aresistor was used to limit the lowering speed of the motor out of thetripped limit switch. In order to obtain adequate dynamic braking whenhoisting into the limit switch a compromise in the value of the resistorwas used, thus limiting the value of retarding torque that could beobtained when lowering out of the tripped limit switch.

In the present invention a rectifier circuit is used in place of theresistor to effectively provide the necessary motor circuit conditionsto insure full dynamic braking when hoisting into the limit switch andto provide increased retarding torque and the resulting slow speed whenlowering out of the tripped limit switch.

In view of the foregoing, one of the objects of the present invention isto provide the necessary motor circuit conditions to insure full dynamicbraking when hoisting into a limit switch and to provide slow speedwhile lowering out of the tripped limit switch.

Another object of the present invention is to provide a motor controlsystem with an improved circuit to connect a direct current motor forrotation in a hoisting direction, to set up a full-powereddynamic-braking circuit when the hoisting movement has passed a certainposition, and to prepare the motor circuit for lowering rotation afterit has passed that certain position.

A further object is to provide a slow speed series motor loweringconnection for lowering a hoist out of an overhoi st limit switch zonewithout requiring the use of an additional electromagnetic switch orrelay, or an addito the voltage across the rectifier, the resistancebeing higher when the voltage is low. Thus the rectifier cir-' cuitconstitutes a high resistance, if not effectively an open circuit,shunting the armature when the voltage across the armature is low tocause the armature to rotate at a slow speed with an unloaded hook.Further, the resist: ance of the rectifier circuit shunting the armaturedecreases as the yoltage across the armature increases, thereby limitingthe armature rotational speed to a .safe speed and in the event of anoverhauling hook load. The system further includes suitable switchescommon to the rectifier and other circuits to selectively energize thosecircuits so that an operator has adequate and safe control over theoperation of the hoist at all times, even if he has hoisted into a limitswitch. If the operator accidently hoists into a limit switch, the limitswitch is tripped,, a dynamic braking circuit is established to retardhoisting movement of the armature and the circuits are prepare for safelowering out of the tripped limit switch.

An embodiment of the invention as applied to a hoist is more fullydescribed herein by way of example and not of limitation, thedescription being accompaniedby the attached drawing in which:

' Fig. 1 is a wiring diagram of the hoist motor operating systemincorporating one embodiment of the present in vention;

Fig. 2 is a diagrammatic representation of a motor, limit switch, anddrive used in a hoist; 1'

Fig. 3 is a wiring diagram of an alternate embodiment of the inventionwhich may be substituted for the embodiment incorporated in Fig. 1;

Fig. 4 is a graph illustrating the resistance and currentconductivecharacteristics of a rectifier in its conductive direction.

Referring to Figs. 1 and 2, a direct current motor comprising anarmature 10 having brush terminals 10a and 10b and a series type fieldwinding 11 having terminals 11a and 11b is arranged to be energized fromdirect current power supply conductors 12 and 14 for hoisting tionalcontact on the limit switch and which includes improved means includinga rectifier circuit for eliminating any sudden changes in speed whichwould subjectthe hoist mechanism and motor to severe shocksand strains.

A more specific object is to provide a slow speed series motorconnection in a dynamic braking hoist controller and lowering a loadholding device, such as hook 15 suspended from a cable drum 16 driven bya motor through suitable gearing 18. For operation of the motor ture 10,the field winding 1:1, and the operating winding 21 of a spring appliedelectromagnetically released brake (not shown), and an acceleration anda speedadjusting resistor 22 in series with each other across the supplyconductors 12 and 14. It is noted that the acceleration andspeed-adjusting resistor 22 provides a rheostatically variable voltagesource at the motor and between the line 112 and the field terminal 11bduring at least certain operations of the motor. It is also noted that,during hoisting of the hook 15, current flows through the armature inthe hoisting direction and through the field winding 11 in a forwarddirection.

Overhoisting or hoisting the hook 15 too closely to hoisting 16 causesthe hook block 15' to engage a limit switch 29, which is provided with apair ot normally closed contacts 29a and 29b and a pair of normally opencontacts 29c and 29d. Hoisting the hook block 15' into the limit switch29 causes the limit switch 29 to open the normally closed contacts 29aand 29b and to close the normally open contacts 29c and 29d.

Referring again to Fig. 1, contacts 29a are in series with contacts 19m,contacts 29b interconnect terminals 10b and 11a, and contacts 290interconnect terminals 10a and 11a. Opening of the contacts 291: of thelimit switch 29 disconnects the armature terminal 10a from the supplyconductor 12, and opening of the contacts 29b interrupts the usualconnection between the armature terminal 10b and the field terminal 11a.Closure of the contacts 290 and 29d completes a dynamic braking circuitfor the motor from the armature terminal 10a through the contacts 290 tothe field terminal 11a, thence through the field winding 11 from theterminal 11a to the terminal 11b, thence through a rectifier R1, aresistor 37 from one end 37a to its other end 37b, and finally throughthe contacts 29d, to the armature terminal 10b. During dynamic braking,current flows through the armature 10 in the lowering direction andthrough the field winding 11 in the forward direction. A rectifier R2 isconnected between a tap 37 on resistor 37 and the terminal 11a. Duringdynamic braking, occasionedby tripping of the limit switch 29, therectifier R2, because it blocks current flow therethrough in a reversedirection, i.e. from terminal 11a to tap 37, insures that all of thedynamic braking current from the armature 10 passes through the fieldwinding 11, thereby providing full dynamic braking torque for givenvalues of armature current, as determined by the resistance of theresistor 37.

-In order to lower the hook while the limit switch is in its trippedposition, at which time contacts 29a and 29b are open and contacts 29cand 29d are closed, a control circuit including a master switch 54 isprovided to energize electromagnetic switches having coils 20 and 40which close contacts 20m and 40m, respectively. This completes a motorcircuit from the supply conductor 12 through the contacts 40m, thearmature 10 from the terminal 10*!) to the terminal 10a, the limitswitch con tacts 290, the field winding 11 from the terminal 11a to theterminal 11b, the brake winding 21, all or a part of the resistor 22,and the contacts 20m to the supply conductor 14. The motor is nowconnected as a series machine and exerts a torque in the loweringdirection wherein the armature 10 rotates in the lowering direction andthe current flows through the armature 10 in lowering direction andthrough the field winding 11 in the forward direction.

A unidirectional or rectifier circuit around the armature 10 from thearmature terminal 10b to the field terminal 11a through the limit switchcontacts 29d and a portion of the resistor 37 from end 37b to tap 37'and the heretofore referred to unidirectional resistor or rectifier R2,in its forward or current conductive direction also exists during seriesmotor lowering. This rectifier circuit reduces the motor torque anamount depending upon the resistance value of the rectifier circuit.

Rectifier R2 and the portion of resistor 37, between 37' and end 37bhave an ohmic value small enough in the current conductive direction toserve efiectively as an armature shunt resistor in lowering out of thelimit stop and still do not effect the dynamic braking torque whenentering the limit switch zone. Further, the rectifier R2, by itsinherent characteristics as will be later described more fully, is ahigher resistance in its conductive direction when the voltage acrossthe armature is low than when the voltage across the armature is high.This rectifier circuit, including rectifier R2, being connected betweenthe terminals 10b and 11a, provides an armature shunt which limits thelowering speed to a safe value during lowering operations with the limitswitch 29 tripped.

It is to be noted that when the limit switch contacts 2911 are open, acircuit consisting of rectifiers R1 and R2, and a portion of resistor 37is connected in shunt with series field winding 11 in such manner thatcurrent will not flow through the rectifiers from the terminal 11a to11b so that the rectifiers do not have to conduct heavy load currentsduring normal operations of the motor. Thus, much smaller and lighterweight rectifiers may be safely used since they will only conductcurrent while the limit switch 29 is tripped.

Fig. 4 shows the forward or conductive characteristic of a typicalselenium rectifier such as is used for rectifier R2. It will be notedthat no appreciable current will flow until a certain voltage is presentacross the rectifier stack. As the voltage is increased beyond thiscertain voltage, the current conducted through the rectifier risesrapidly. This characteristic of the rectifier insures that a light hookload can be safely lowered at a reasonable rate while still protectingagainst overspeeding with a heavy hook load. This may be explained byconsidering that the rectifier circuit will act as a relatively highresistance armature shunt while the armature voltage is lowcorresponding to a low speed and the rectifier circuit will act as amuch lower resistance when the armature voltage is higher correspondingto'a higher speed. The rectifier must have an inverse voltage ratingsufiicient to withstand the inverse voltage which is present whenentering the limit switch at high speeds. As an example, a desirablerectifier for a 10 hp. 230 volt motor should have about eight standard33 volt cells in series which will provide a momentary blocking voltageof 340 volts and provide a forward characteristic as indicated in Fig.3. The rectifier used in this example has a rectification area of about3 square inches per hp. for each cell and a continuous current ratingabout 20% of the full load motor current.

The herein described and illustrated control system operates as follows:

Hoisting To hoist the hook 15 at one of its speeds, operation of themaster switch 54 energizes the switch coils 19. and 20 to close thecontacts 19m and 20m to complete a circuit extending from the conductor12 through the contacts 19m, the contacts 29a, the armature 10, in thehoisting direction, the contacts 2%, the field winding 11 in the forwarddirection, the braking winding 21, the resistor 22 and the contacts 20mto the conductor 14. It is noted that the armature and field are inseries across a source or" voltage from line 12. to terminal 11b whichis rheostatically varied in this instance by resistor 22, and thatcurrent flows in the hoisting direction through the armature and theforward direction through the field to cause rotation of the armature ina hoisting direction. Hoisting of the hook 15 at other speeds may beobtained in the same manner as described in the referred to patent.

Hoisting into limit switch If the hook is hoisted high enough so thatthe limit switch 29 is tripped, the contacts 29a open to disconnect themotor from the power conductor 12, and thus deenergize it, the contacts29b open, and the contacts 290 and 29d close to complete the previouslytraced emergency dynamic braking circuit. Dynamic braking currentgenerated by continued hoisting rotation of the armature now flowsthrough armature in the lowering dimotion from 10b to 10a, thencethrough contacts 290, field winding 11 in the forward direction from 11ato 11b, through rectifier R1 in its conductive direction, resistor 37and contacts 29d, to terminal 10b. Full dynamic braking current asgenerated by the armature 10 flows through the field winding sinceunidirectional resistor or rectifier R2 is blocking the flow of any partof the dynamic braking current therethrough and the rotation of thearmature in the hoisting direction is brought to a fast, safedeceleration and smooth stop.

Lowering while limit switch is tripped The hook may be lowered while thelimit switch 29 is tripped. Lowering hook 15 while the limit switch 29is tripped is accomplished by moving the master switch 54 into any ofits lowering positions, thereby effecting connection of the motor as aseries machine rotating the armature in the lowering direction andexerting a downward torque. With the limit switch 29 tripped, that is,with contacts 29a and 2% open and contacts 29c and 29d closed, closureofthe contacts 40m and m completes a motor circuit from the supplyconductor 12 through the contacts 40111, the armature winding 10 in thelowering direction from 10b to 10a, the now closed contacts 29c and thefield winding 11 in the forward direction from the terminal 110 to 11b.It is particularly noted that during rotation of the armature '10 in thelowering direction the current flow therethrough is in the samedirection (from 10b to 10a) as the generated dynamic braking currentwhich stopped the hoisting movement of the armature, and that thecurrent through the field 11 is in the normal forward direction (from11a to 11b). The resistor 22 provides a variable voltage source be tweenline 12 and terminal 11b.

Simultaneously, an armature shunt circuit is formed in which current mayflow through contacts 29d, a portion of resistor 37 from end 37b to tap37', and rectifier R2 in its conductive direction to the terminal 11a,This circuit constitutes a rectifier circuit shunting the armature 10and the closed contacts 290 to limit the speed of the armature. At thistime rectifier R1 is blocking current flow therethrough. The rectifiercircuit just described produces a slow lowering speed while the limitswitch is tripped, providing a strong field and an armature shunt. Toobtain adequate retarding action, the resistance of the rectifiercircuit including rectifier R2 and the associated portion of resistor 37between contacts 29d and tap 37' should be very low.

Continued lowering of hook 15 will reset the limit switch 29. If,through some failure, the limit switch 29 does not return to its normalposition, the hook 15 may be lowered as far as desired. Thus when thesystem is used in a hoist, a heavily or lightly loaded hook 15 may belowered safely to the ground even though the limit switch 29 remains inits tripped position.

If, as it is supposed to, the limit switch 29 returns to its normalposition when the hook is moved therefrom, i.e. contacts 29a and 2%close while contacts 290 and 29d open, continued lowering or movement inthe reverse direction is obtained through normal lowering operation ofthe master 54 as described in the referred to patent.

An alternate embodiment of the invention is obtained by substituting thecircuit of Fig. 3 for its corresponding portion of the circuit ofFig. 1. In both figures like characters represent the same parts. InFig. 3, the rectifier R2 is connected in shunt with contacts 2% anddirectly between terminals 10b and 11a and rectifier R1, is eliminated.Basically, the alternate circuit of Fig. 3 Operates in the same mannerand for the same purpose 6 as the circuit of Fig. 1. However, duringlowering of the hook 15 while the limit switch 29 is tripped, rectifierR2 is directly in shunt with the armature and without any resistance inseries with the rectifiertRZ. Also, since rectifier R1 is omitted,resistor 37 is connected directly between contacts 29d and terminal 11b,and provides a shunt circuit around rectifier R2 and field winding 11.However, and because of the usual large ohmic value of resistor 37, thisshunt circuit has practically no etfect on the operation of the systemduring lowering with a tripped limit switch and while contacts 29d areclosed and contacts 29!; are open.

Having thus described my invention, I claim:

1. A motor operating system comprising a source of direct currentenergy, an armature, a field winding for the armature, a rectifieradapted to conduct current in a forward direction and block current flowin a reverse direction, a resistor, a first circuit when energized bysaid source directing current through said armature in a hoistingdirection and through said field winding in a forward direction inseries to cause hoisting rotation of the armature, a second circuiteffective upon deenergiza tion of said first circuit to direct currentgenerated by continued rotation of said armature after suchdeenergization through said armature in a lowering direction and throughsaid field winding in the forward direction and through said resistor todynamically stop rotation of said armature, a third circuit whenenergized by said source directing current through said armature in alowering direction and through said field winding in said forwarddirection and including a rectifier circuit shunting said armature andincorporating said rectifier, said rectifier circuit constituting avariable resistance which decreases in magnitude as the voltage acrossthe armature increases in magnitude, and switch means common to saidrectifier and first and second and third circuits to selectively controlsaid circuits.

2. A motor operating system comprising a rheostatic controlled variablevoltage source of direct current energy, an armature, a field winding, arectifier adapted to conduct current in a forward direction and blockcurrent flow in a reverse direction, a resistor, a first circuit whenenergized by said source directing current through said armature in ahoisting direction and through said field winding in a forward directionin series to cause hoisting rotation of the armature, a second circuiteffective upon deenergization of said first circuit to direct currentgenerated by continued rotation of said armature after suchdeenergization through said armature in a low ering direction andthrough said field winding in the forward direction and through saidresistor to dynamically stop rotation of said armature, a third circuitwhen energized by said source directing current through said armature ina lowering direction and through said field winding in said forwarddirection and including a rectifier circuit shunting said armature andincorporating said rectifier, said rectifier circuit constituting a highresistance shunting said armature when the voltage thereacross is lowand constituting a low resistance shunting said armature when thevoltage thereacross is high, and switch means common to said rectifierand first and second and third circuits and selective to control theenergization of said first and third circuits, to render said selectedcircuit effective, and including the control of the energization of saidthird circuit while the second circuit is effective.

3. A motor operating system for a direct current motor having anarmature and a field winding, said system comprising a source of energy,a first normally open switch and second normally closed switch and themotor armature and a third normally closed switch, and the field windingelectrically connected in series in that order across said source ofenergy, a fourth normally open switch shunting said armature and saidthird switch, a fifth normally open switch and a resistor having firstand second resistance portions shunting said third switch and said fieldwinding, a rectifier circuit shunting said third switch and including arectifier and said first portion of said resistor and said fifth switch,and a sixth normally open switch shunting said first and second switchesand said armature, said first switch being closable to connect the motorto the source of energy to cause current to flow in one directionthrough said armature and field winding in series to rotate the armaturein a hoisting direction, means to open the second and third switches todisconnect the motor from the source of energy and to close the fourthand fifth switches to cause current generated by continued rotation ofthe armature after such disconnection to flow through said fourth switchand said field winding and said resistor and said fifth switch todynamically brake the motor, said sixth switch being closable while saidfourth and fifth switches are closed to connect said motor to saidsource of energy to cause current to flow through said fifth switch andsaid first portion of said resistor and said rectifier and said fieldwinding in said one direction and to flow through said armature and saidfourth switch in another direction to rotate the armature in a loweringdirection after hoisting rotation thereof has stopped.

4. The structure of claim 3 wherein said means is responsive to rotationof said armature in said hoisting direction.

5. The structure as defined in claim 3 wherein said source of energy isa rheostatically controlled variable voltage source of energy andwherein said rectifier and said first portion of said resistoreffectively constitutes a high resistance shunting said armature whenthe voltage thereacross is low and constitutes a low resistanceeffectively shunting said armature when the Voltage thereacross is high.

6. A motor operating system for a direct current motor having anarmature and a field winding, said system comprising a source of energyand a first normally open switch and a second normally closed switch andthe motor armature and a rectifier and the field winding electricallyconnected in series in that order across said source of energy, saidrectifier being connected to pass current in a forward direction fromsaid armature to said field winding and to block passage of current in areverse direction from said field winding to said armature, a thirdnormally open switch shunting said armature and said rectifier, a fourthnormally open switch and a resistor shunting said rectifier and saidfield winding, and a fifth normally open switch shunting said first andsecond switch and said armature, a sixth normally closed switch shuntingsaid rectifier, said first switch being closable to connect the motor tothe source of energy to cause current to flow in a hoisting directionthrough said annature and in a forward direction through said fieldwinding in series to rotate the armature in a hoisting direction, meansto open the second and sixth switches to disconnect the motor from thesource of energy and to close the third and fourth switches to causecurrent generated by continued rotation of the armature after suchdisconnection to flow through said armature in a lowering direction andsaid third switch and said field winding in said forward direction andsaid resistor and said fourth switch to dynamically brake the motor,said fifth switch being closable while said second and sixth switchesare open to connect said motor to said source of energy to cause currentto flow through said rectifier and said field winding in said forwarddirection and to cause current to flow through said armature in saidlowering direction to rotate the armature in a lowering direction afterforward rotation thereof has stopped.

7. The structure according to claim 6 wherein said means is responsiveto rotation of said armature in said hoisting direction.

8. The structure as defined in claim 6 wherein said source of energy isa rheostatically controlled variable voltage source of energy andwherein said rectifier eifectively constitutes a high resistanceshunting said armature when the voltage thereacross is low andconstitutes a low resistance effectively shunting said armature when thevoltage thereacross is high.

9. In a motor and control system combination for raising and lowering aload, a direct current motor adapted for connection to a source of powerand having an armature winding and a field winding, which windings areadapted for shunt and series connection, selectively, with each other,for operation of the motor as a shunt and series machine, selectively, alimit switch mechanism having a normally closed contact means connectedbetween a first terminal of said armature winding and a first terminalof said field winding and having a normally open contact means connectedbetween a second terminal of said armature winding and said firstterminal of said field winding, and being responsive to an elevatedposition of the load while the load is being raised to open saidnormally closed contact means and to close said normally open contactmeans, means for connecting a second terminal of said field winding toone side of said source of power, means for connecting said secondterminal of said armature winding to the other side of said source ofpower for hoisting a load, and means for connecting said first terminalof the armature winding to said other side of said source of power forlowering a load when the second field terminal is connected to said oneside of said source of power and while said normally closed contactmeans is open and said normally open contact means is closed, anacceleration resistor connected in series with said motor when saidsecond armature terminal is connected to said source of power, theimprovement which comprises a rectifier connected in parallel with saidnormally closed contact means and between said first armature terminaland said first field terminal to provide an armature shunt connectioncooperating with said acceleration resistor to limit the lowering speedof the motor while said normally closed contact means is open and saidnormally open contact means is closed and until said normally closedcontact means recloses.

References Cited in the file of this patent UNITED STATES PATENTS

